Purpose Acetylene (C 2 H 2 ) is employed for the quantification of important biological processes such as nitrogen fixation, nitrous oxide reduction, ammonium and methane oxidation, and methanogenesis. Although acetylene is not a natural product, the ability of bacteria to grow on C 2 H 2 is a phenomenon common to soils and sediments. Our experiment was designed to study the modification of CO 2 production, O 2 uptake and microbial biomass (C mic ) in soil in response to the consumption of added acetylene. Materials and methods Two soils (peat-muck and Eutric Cambisol) were incubated under well aerated (60% water holding capacity [WHC]) or flooded conditions, and enriched with C 2 H 2 in the range 0.1-10 kPa (initially, 0.434-63.4 mmol C 2 H 2 kg −1 ) at a constant temperature of 20°C. Gases were measured chromatographically, while C mic was measured by the physiological SIR (substrate-induced respiration) method based on the initial response of microorganisms to glucose amendment. We used a simple calculation of net CO 2 , net O 2 and net C mic (as differences between amended and not amended soils) to estimate the contribution of C 2 H 2 to the total respiration and microbial growth during the incubation. Results and discussion Peat-muck soil consumed more C 2 H 2 than Cambisol (maximum 54.03 vs. 19.25 mmol kg −1 , p<0.001). Acetylene utilization was faster and larger in flooded than in wet soils (16.2 and 7.81 mmol kg −1 , respectively, p<0.05), and followed the tendency observed for C mic . Depending on C 2 H 2 enrichment, air-water conditions, and soil tested, both reduction and stimulation of measured activities were observed in response to acetylene consumption. Low C 2 H 2 uptake, especially in Cambisol incubated at 60% WHC, resulted in the reduction of soil respiration and biomass (by 1-29%). Large C 2 H 2 consumption in flooded soils stimulated CO 2 production, O 2 uptake and C mic , even by 78%, 72% and 43%, respectively. Net CO 2 , net O 2 , and net C mic were linearly positively related to the quantity of consumed C 2 H 2 (p<0.001).Conclusions Acetylene utilization was a combined effect of initial C 2 H 2 , soil properties, and air-water status. The amount of consumed C 2 H 2 was the highest in flooded peat-muck soil enriched with 10 kPa, and lowest in Cambisol at 60% WHC with 0.1 kPa C 2 H 2 . Consumption of 1 mol acetylene induced production of 0.5 mol CO 2 , and uptake of 0.4 mol O 2 . Low acetylene consumption (<6 mmol kg −1 ) resulted in reduction of both soil respiration and microbial biomass.
